This invention relates generally to fluid pressure vessels which incorporate non-metallic liners, and in particular to a new and improved system and structure for attaching bosses used in such vessels to the liners.
Composite (fiber reinforced resin matrix) containers or vessels have come into common use for storage of a variety of fluids under pressure, including storage of oxygen, natural gas, nitrogen, rocket fuel, propane, etc. Such composite construction provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. This combination of lightness in weight and resistance to failure is possible due to the high specific strengths of the reinforcing fibers or filaments (carbon, glass, aramid, etc.) which, in the construction of pressure vessels, are typically oriented in the direction of the principal forces.
Since the resin matrix of the composite pressure vessel (shell) is subject to cracking and crazing during service and use, the vessels are oftentimes furnished with fluid impermeable liners. While metal liners are most common, elastomeric rubber and thermoplastic liners have also been utilized. Advantageously, the liners are designed not only to prevent leaks from the vessel, but also to serve as mandrels during the vessel fabrication--i.e., profile definition for the composite shell.
Maximum structural efficiency is attained when the lightweight composite shell is used to carry the majority of the load, with little contribution from the liner; and so if metal liners are used, the liner must be relatively thin in order to reduce the weight. However, thin metal liners have low fatigue life and because of this problem of sacrificing a low weight for durability, and vice versa, users have increasingly looked to use non-metallic liners which are lighter in weight and yet fluid impervious.
One problem with the use of non-metallic liners is that of securely attaching the liners to the vessel bosses which are typically metallic. The end-bosses support fluid passage into and out of the vessel and also may function in the fabrication of the composite shell by providing for fiber turnaround at the ends or poles of the vessel and for mandrel support if filament winding is used to construct the shell. When metal liners are used, such bosses are generally constructed integrally with the liners, but with the increased use of non-metallic liners, other methods of attaching the bosses to the liners have been needed. Some prior approaches to attaching non-metallic liners to bosses have included adhesive-bonding of the boss to the liner (if the liner is sufficiently rigid), and simple reliance on the internal pressure in the vessel to provide a boss-liner seal (if the liner is a flexible, collapsible membrane). FIGS. 2 and 3 of the drawings illustrate these two approaches for attaching non-metallic liners to bosses, and will be discussed momentarily.
The above two approaches, however, present problems including breakdown under fatigue cycling or exposure to certain environments of the metal (boss) to non-metallic (liner) bonding, disintegration of the bond because of significantly different coefficients of thermal expansion of most metals versus non-metals, and shifting of the liner in the shell (if only internal pressure is relied upon) giving rise to leak paths through the boss-liner joint.
In addition to the need for properly attaching the liner to the bosses, it is also important to provide a shear layer at the composite shell/metallic boss interface to reduce the tendency for large shear stresses to develop at that interface. In the past, rubberized compounds have been utilized for such layers but these are prone to disintegrate over time.